As will be appreciated herein below, except as otherwise indicated, alloy designations and temper designations refer to the Aluminum Association designations in Aluminum Standards and Data and the Registration Records, as published by the Aluminum Association in 2010 as is well known in the art.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
AlMg alloys, and in particular AlMgSc alloys, are suitable candidates for aerospace applications due to their low density compared to various existing aluminium alloys, while at the same time the strength and toughness level are comparable. However, the aerospace applications require the sheet to be formed to complex curved shapes, such as fuselage skin, lower wing skin, upper wing skin or wing stringers. Currently, creep forming is the preferred method for forming aluminium alloy sheet of the 5000-series. During creep forming, the sheet is heated in an autoclave to a temperature typically above about 300° C., and a load is applied to the sheet, for example by using a vacuum to draw the sheet into the mould. During the process, the sheet slowly deforms to the desired shape, and which may take several hours. The main advantage of this forming process is the high shape accuracy, and that it can be combined with laser beam welding of the stringers to the sheet. Disadvantages are the high capital costs of the creep anneal installation, and the long forming times required.
An alternative forming process known in the art is stretch forming, in which the sheet is gripped at its margins and stretched over a mould. This forming technique is used for age-hardenable aluminium alloys in the aerospace industry. However, when using stretch forming for 5000 alloys, the Portevin-Le Chatelier (“PLC”) effect results in so called PLC-bands on the formed panel. These are parallel bands appearing on the surface of the formed sheet due to an inhomogeneous flow during stretching, visible also from the serrated stress-strain curves recorded during the stretch form process. Such PLC bands are considered to be unacceptable surface defects and so far prevent the use of such panels for the aerospace industry or in automotive applications.
One possibility to prevent PLC band formation is to reduce the temperature during stretch forming to cryogenic temperatures. This method has been disclosed in patent document U.S. Pat. No. 4,159,217, where it was proposed to stretch-form a work-hardened sheet at cryogenic temperatures in the range of −100° C. to about −200° C. The sheet was cooled down by immersing in a suitable cryogenic medium such as liquid nitrogen, or in a mixture of dry ice and alcohol. However, U.S. Pat. No. 4,159,217 is silent on the tensile properties and thus the feasibility of stretch forming at low temperatures for 5000-series alloys. Furthermore, the temperatures used are very low, requiring copious use of cryogenic media.
It is therefore an object of the invention to provide a process for forming shaped aluminium alloy panels, which provides good results for 5000-series alloy sheet, and which is more cost-effective than the disclosed prior art method. In addition, it is an object of the invention to provide shaped aluminium alloy panels of the 5000 alloy series which have good combinations of elongation, tensile properties and corrosion resistance after forming.